Anisotropic oxygen diffusion properties in Pr₂NiO _4+δ and Nd₂NiO_4+δ single crystals

The anisotropy of the oxygen anionic conductivity was measured for two mixed ionic electronic conducting (MIEC) oxides with the 2D K ₂NiF₄-type structure, i.e., Nd₂NiO _4+δ and Pr₂NiO_4+δ, using high quality single crystals. Measurements of the oxygen diffusivity and surface exchange performed parallel and perpendicularly to the [001] direction, from 450 to 700 C, using the isotope exchange depth profile (IEDP) technique, combining ¹⁶O/¹⁸O exchange and secondary ion mass spectroscopy (SIMS) are reported. For both materials the diffusion is about 3 orders of magnitude higher along the (a,b)-plane compared to the perpendicular (c-axis) direction. These values are among the highest when compared to several state-of-the-art MIEC materials. The diffusion along the (a,b)-plane for Pr ₂NiO_4+δ is higher than that of Nd ₂NiO_4+δ due to a much lower diffusion activation energy (0.5 and 1.4 eV for Pr₂NiO_4+δ and Nd ₂NiO_4+δ, respectively). A large anisotropy is also observed in the surface exchange coefficient (k*) values for both materials, with (a,b)-plane coefficients being 1 to 1.5 orders of magnitude larger than those for the c-axis.